1. Field of Invention
The present invention relates to a laser pulse measurement technique, and more specifically to a method and system for detecting single-shot pulse contrast based on fiber array.
2. Description of Related Arts
Any discussion of the prior art throughout the specification should in no way be considered as an admission that such prior art is widely known or forms part of common general knowledge in the field.
Focused intensity in excess of 1020 W/cm2 now can be reached by high-power laser systems based on chirp pulse amplification, which may find many applications such as plasma physics, high-order harmonic generation, inertial confinement fusion, or quantum electrodynamics. In the systems with such high intensities, however, reflections, scattering, amplified spontaneous emission, or incomplete temporal compression may result in considerable pedestal or pre-pulses and post-pluses. The contrast, defined by the ratio of the peak intensity of the main pulse to its background, especially in the leading part, is therefore one of the most important parameters of a high-power laser pulse. For example, the absolute intensity of the long pedestal or pre-pulse must be under the threshold of pre-plasma generation for a clean light-matter interaction experiment. The higher the peak intensity, the higher the contrast is required. This, of course, necessitates a more delicate design of the laser system and/or pulse's cleaning technique for higher peak intensity to meet the experiment criteria. On the other hand, it also challenges the diagnostic technique for pulse contrast, which is necessary for experimental data analysis and system optimization.
Current diagnostic techniques for pulse contrast are typically based on nonlinear correlation, in which a clean reference is obtained by second-harmonic generation (SHG) of the to-be-characterized pulse. Sum-frequency generation (SFG) or difference-frequency generation (DFG) may serve as the nonlinear process for correlation, and the dependence of the third harmonics or the idler on temporal delay is measured to reveal the information of the shape and contrast of the primary pulse. One of the most important specifications of such an instrument is its dynamic range, which determines the ability to distinguish the utmost difference of the signal levels. For a specific detection system the strongest signal detected must be kept below the saturation level, which is accomplished commonly by attenuation using a calibrated neutral density filters(s) [NDF(s)]. At the low end, the noise level sets the bottom limit of the detectable signal. The noise level sets the bottom limit of the detectable signal. The noise limit is indeed the most severe problem that restricts the obtainable dynamic range. Stray light suppression and using detector with low noise and high sensitivity are thus preferable. In a time-scanning configuration for characterizing repetitive pulses, a lock-I amplifier or boxcar averager may be used to increase the dynamic range by 1-2 orders of magnitude. In addition, a larger dynamic range could be achieved in principle by further increasing the intensity of the input beam while increasing the attenuation of the signal peak.
Although a dynamic range in excess of 1011:1 has been demonstrated for repetitive millijoule input pulses in a time-scanning mode, it is typically about 106-107:1 for single-shot technique, which is especially desirable for high energy/high-power laser systems operating at extremely low repetition rate or even non-repetitively, the information of the shape and contrast is obtained from an isolated pulse. Third-order correlation based on SHG-SFG cascaded processes is widely used. Variation of the time delay is realized by intersecting the two interacting beams and/or tilting the pulses onto the SFG crystal, and the correlation in time is then transformed into a spatial intensity distribution. Currently the pulse correlation trace in a single-shot measurement is recorded by a multi-element detector capable of parallel measurement such as a diode array. To increase the measurable contrast a calibrated attenuator is normally positioned to reduce the central portion of the signal while leaving the low-level pedestal un-attenuated. The poorer dynamic range in a single-shot mode, when compared to that of a time-scanning mode, is largely due to the performance limits in its detection system. The attenuator for the central signal reduction, the width of which is narrower than that of the signal beam, will inevitably introduce extra noise through edge scattering or diffraction. Furthermore, the diode array for parallel measurement is much less sensitive than a photomultiplier tube (PMT) in a time-scanning setup, particularly when techniques for improving the signal-to-noise ratio, such as lock-in amplification or boxer averaging, are precluded in a single-shot measurement.